The present invention relates to a network analysis device, system and methodology, and more particularly, to a network measurement device and a method that evaluates physical characteristics of network cables in response to a single to determine attenuation of open or shorted network cables from a single end of the network cables.
This invention relates generally to cable testing and troubleshooting and in particular to measuring the attenuation characteristics of cables.
Local Area Networks (LANs) include a large number of interconnected computers, work stations, printers, and file servers and other devices. A LAN is commonly implemented by physically connecting these devices with copper-conductor, twisted-pair LAN cables, the most common being an eight-wire cable which is configured in four twisted-wire pairs (commonly referred to simply as xe2x80x9ctwisted pairsxe2x80x9d) within a flexible wrapper that may include an electrostatic shield, with each end of the cable terminated in an industry-standard connector or terminator. A LAN that has been poorly installed, or has faulty cables or connectors, can result in data transmission errors. Therefore, the LAN must be tested to verify proper operation and performance.
Attenuation limit is a specified performance parameter that indicates the maximum signal loss allowable in a network system. Attenuation is the decrease in the power of a signal as it propagates along a cable. Alternatively, it may result from signal loss through a faulty connector or damage to the cabling. If the attenuation exceeds a specified limit, the network is not in compliance with performance specifications.
Traditionally, cable attenuation is calculated through measurement of the ratio between input and output amplitudes at different frequencies. A conventional methodology for measuring attenuation requires access to both ends of the cable under measurement. A standard configuration to implement this methodology typically involves a main testing/troubleshooting unit and a remote unit coupled via industry-standard connectors to opposite ends of the cable. The remote unit measures the signal at the other end of the cable in response to each stimulus signal received from the main unit positioned at the opposite end of the cable. The signal source is incremented in discrete steps across a range of frequencies, while making measurements of the input and output amplitudes at different frequencies. A ratio between the input and output amplitudes at different frequencies is compared with a worst-case attenuation versus frequency function specified for the cable under measurement to determine network compliance or that the attenuation limit for the cable has not been exceeded. However, this conventional method requires complex testing units that are coupled to both ends of the cable under measurement, which impedes cable testing after network installation.
The present invention is designed to overcome limitations in the prior art by providing a measurement and modeling approach that calculates cable attenuation from a single end location on a cable under measurement. Alternatively, the measurement device is located at a single non-end or intermediate location on a cable, between the network device terminating one end and the node terminating the opposite of the cable. The present invention discloses a system, device and methodology to calculate attenuation in network cables, simultaneously or separately, without the need to access and attach to both ends of the cable under measurement with apparatuses to produce the attenuation measurements. The far end of the cable is electrically open or shorted.
To implement the invention, a measurement device injects and samples responses to time domain reflectometer (TDR) signals. In one aspect of the invention, calibration waveforms, in terms of a voltage drop over a defined time domain, are sampled by a digitizer of the measurement device, recording the response of the signal exposed to known impedances connected to the measurement device. The present invention uses three calibration waveforms, namely VT(t), VS(t), and VO(t), to calibrate a model that estimates internal parameters inherent to a current source responsible for generating the TDR signal in a signal generator. In a second aspect of the invention, the actual cable under measurement is injected with a TDR signal and the response of the cable to the signal and its reflection from the open or shorted far end is sampled by the digitizer and recorded as a cable waveform, VC(t). VC(t) represents the voltage drop across the measurement or near end of the cable in response to the propagating TDR signal as a function of time.
Next, a model estimating the current source is calibrated using the calibration waveforms. The calibrated model estimates parameters A(f), B(f), and D(f) that represent the internal parameters of a current source used to generate the TDR signal. Using the modeled parameters of the current source and the cable waveform, impedance of the cable, ZC(t), is calculated pursuant to the model. From this information, attenuation, H(f)db of the cable under measurement is calculated and displayed as a function of frequency. In some cases, the attenuation of the cable requires further evaluation because of low signal-to-noise effects at high frequencies in long cables. Under such circumstances, the invention employs an additional model that fits the calculated attenuation of the cable at low frequencies to correct for abnormalities resulting in the attenuation data at high frequencies.